1. Field of the Invention
Exemplary embodiments of the invention relate to a backlight unit capable of local dimming and a liquid crystal display using the same.
2. Discussion of the Related Art
A liquid crystal display according to an active matrix driving method displays moving pictures using thin film transistors (TFTs) as switching elements. The liquid crystal display is applied television sets as well as portable information devices, office automation apparatuses, computers because it is possible to be made thinner than a cathode ray tube (CRT). Accordingly, the liquid crystal display rapidly replaces the CRT in a recent.
The liquid crystal displays have an increasing application range because of light-weight, thin, low-power consumption features. The liquid crystal display has been used for portable computers such as laptop computers, office automation apparatuses, audio/video devices, and indoor/outdoor advertisement displays. A transmissive type liquid crystal display which occupies all most liquid crystal display devices displays a desired picture by controlling an electrical field applied to liquid crystal layer to modulate light-incedent from a backlight unit. The backlight unit is mainly classified into a direct type and an edge type.
Picture quality of the liquid crystal display depends on contrast property. The method improving the picture quality by modulating light-incedent from the backlight unit has a limitation. In order to improving the contrast property, a backlight dimming control method which adjusts luminance of the backlight unit according to the picture has been variously attempted. The backlight dimming control method may reduce power consumption by adaptably adjusting the luminance of the backlight unit depending on an input picture. The backlight dimming control method includes a global dimming method entirely adjusting luminance of a display screen and a local dimming method locally adjusting luminance of the display screen. The global dimming method may improve dynamic contrast measured between a previous frame and a next frame but it is difficult to improve static contrast. On the other hand, the local dimming method may improve static contrast by locally controlling luminance of the display screen in one frame period. However, the local dimming method needs a very complicated construction and algorithm because it makes luminance differences and flickers between a plurality of blocks divided in the display screen and flicker and also it needs many circuit elements. Also, the local dimming method is applicable to only the liquid crystal display using the direct type backlight unit.
The edge type backlight unit has a construction in which a light source is arranged to face a side surface of a light guide plate and a plurality of optical sheets are disposed between a liquid crystal display panel and the light guide plate. The edge backlight unit may be implemented to have a structure thinner than the direct type backlight unit. However, it is impossible to apply the local dimming method to of the edge type backlight unit because the light source illustrates one side of the light guide plate and the light guide plate serves to convert a spot light source or a line light source into a surface light source.
On the contrary, the direct type backlight unit has a construction in which a plurality of optical sheets and a diffusion plate are disposed under the liquid crystal display panel and a plurality of light sources are disposed under the diffusion plate. Even though the direct type backlight unit implements the local dimming method because the plurality of light sources are disposed under the diffusion plate and can be independently controlled, it is difficult to reduce a thickness thereof. Accordingly, it causes a problem to make a slim design of the liquid crystal display difficult. The reason why the direct type backlight unit can not be made to a slim construction is a space which is necessarily formed between the light sources and the diffusion plate. The diffusion plate diffuses light-incedent from the light sources to make luminance of the display screen uniformly. In order to sufficiently diffuse light-incedent from the light sources, the space between the light sources and the diffusion plate should be sufficiently guaranteed. According to the trend requiring the slim liquid crystal display, the space between the light sources and the diffusion plate is narrowed in gradual, but it makes bright lines on the display screen to lower a luminance evenness of the display screen because the light sources are observed on the display screen when the light from the light sources can not be sufficiently diffused. In order to resolve the problem about the luminance evenness of the display screen, there have been proposed many resolutions such as a method increasing number and arrangement density of the light sources, a method enforcing a diffusing function by forming minute patterns or lens patterns on the diffusion plate facing the liquid crystal display panel and so on. However, the above methods also may cause an increased cost and have definite limitations in respect of improving the diffusion of light.